Endocrine disorders in pregnancy - thyroid disease
Description
Physiological changes - why the reference ranges move
- inc hCG -> weak TSH-receptor agonism (shared alpha subunit) -> TSH falls, fT4 rises in T1
- Peaks ~8-14 weeks, mirroring hCG
- inc oestrogen -> inc TBG -> inc total T4/T3 (use free hormones, or total T4 x 1.5)
- inc renal iodide clearance + fetal transfer -> inc iodine requirement ~50%
- Placental type 3 deiodinase inactivates T4/T3 -> inc maternal demand
- Thyroxine requirement rises 25-50% by ~week 8 in a treated hypothyroid woman
What crosses the placenta
| Crosses | Does not cross |
|---|---|
| TSH-receptor antibodies (TRAb), TPO and Tg antibodies | TSH |
| Thionamides (carbimazole, PTU) | hCG |
| TRH | Maternal thyroglobulin |
| Iodine and radioiodine | |
| Thyroxine - only in small amounts |
- *This table explains everything* - fetal Graves from maternal TRAb even after maternal thyroidectomy; fetal goitre from maternal thionamide; maternal T4 cannot rescue fetal hypothyroidism
- Fetal thyroid does not function until ~12 weeks - first-trimester neurodevelopment depends entirely on maternal T4
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